1. Field of the Invention
The present invention relates to a micro vacuum pump for maintaining vacuum in a chamber and an apparatus including the same. And, more particularly, the present invention relates to a micro vacuum pump that is capable of maintaining a high degree of vacuum, enhancing exhaust performance, and securing quality over an extended period of time.
2. Description of Related Art
Most apparatuses requiring a vacuum environment employ diverse exhausting methods to enhance the degree of internal vacuum. For example, there are semiconductor manufacturing apparatuses incorporating deposition treatment units, dry etching units, etc., or surface observing apparatuses incorporating electron microscopes, etc. These apparatuses employ ion pumps or turbo-molecular pumps or other types of vacuum pumps that are large and provide high exhausting speed to exhaust the interior of the vacuum chambers of the apparatuses at all times thereby to maintain a high degree of vacuum.
Vacuum airtight apparatuses such as cathode ray tubes (CRTs) or flat panel displays do not carry out regular exhaust by large, expensive vacuum pumps because they are required to achieve reduced size and weight and lower cost. In the vacuum airtight apparatuses, getters composed of metal materials such as barium are activated in the vacuum chambers in the vacuum airtight apparatuses to adsorb residual gases so as to maintain substantially the vacuum.
In a CRT, which is one of those vacuum airtight apparatuses, a getter material placed in the tube is evaporated by external high-frequency induction heating or the like so that it adheres to the inner wall of the tube thereby to exhaust any gas in the tube. In this case, the getter material adhering to the inner wall of the tube is chemically active and adsorbs a residual gas, thus enhancing the vacuum in the tube. In a flat panel display also, the vacuum in the display is retained by the adsorption of a residual gas by a getter material as in the case of the CRT.
Hitherto, a micro vacuum pump adapted to secure vacuum in a vacuum chamber by such an exhausting method has been employing a getter device that has been disclosed under a title "GETTER DEVICE AND FLUORESCENT DISPLAY TUBE HAVING THE GETTER DEVICE" in Japanese Unexamined Patent Publication No. Hei 7-29520 (1995).
In the getter device proposed in the publication, protrusions or emitter cones 103 are disposed on a surface of a cathode electrode 102 opposed to a getter 101 so that they face against the getter 101 as shown in FIG. 1. The getter 101 is made of barium or other metal material. The emitter cones 103 are conical. A gate electrode 105 is mounted on a cathode electrode 102 via an insulator layer 104 and provides the surfaces opposed to the getter 101. The gate electrode 105 is provided with holes to be formed around the respective emitter cones 103. The insulator layer 104 also has holes. The gate electrode 105 provides driving forces for the emitter cones 103 to emit electrons.
In this constitution, relative to the cathode electrode 102, a positive potential differences Vp is supplied to the getter 101 serving as the anode and a positive potential differences Vg is supplied to the gate electrode 105. And an electric field is supplied to the emitter cones 103 on the cathode electrode 102. The emitter cone 103 to which the electric field has been supplied emits electrons passing through the hole of the gate electrode 105. The electrons collide against the getter 101 to activate the getter 101. The activated getter 101 develops enhanced reactivity to other atoms and adsorbs gaseous molecules that form the ambient residual gas. This enables the vacuum in the vacuum chamber to be maintained.
Another example that employs a micro vacuum pump is a vacuum airtight apparatus that has been disclosed under a title "VACUUM AIRTIGHT APPARATUS AND DISPLAY DEVICE" in Japanese Unexamined Utility Model Publication No. Hei 7-18341 (1995).
The vacuum airtight apparatus described in the publication is used for a display device that employs a field emission cathode. In this type of display device, an anode electrode 111 that provides a screen has a fluorescent surface 110 on the surface opposed to a cathode electrode 112 as shown in FIG. 2. Relative to the cathode electrode 112, a high potential difference Vp is supplied to the anode electrode 111. For this reason, electrons are emitted from a plurality of protrusions or emitter cones 113 provided to match the pixels on the cathode electrode 112. The emitted electrons pass holes of a gate electrode 115 and a focusing electrode 116 disposed via two insulator layers 114 and the focusing electrode 116 disposed near the anode electrode 111 before they reach the surface of the anode electrode 111. The focusing electrode 116 positioned in the vicinity of the anode electrode 111 is constituted by getter materials. The gate electrode 115 and the focusing electrode 116 are set at potential differences Vg1 and Vg2 respectively and have the almost same potential to that of the cathode electrode 112.
In this configuration, the electrons emitted from the emitter cones 113 collide against the surface of the anode electrode 111 and a gas is sputtered from the surface of the anode electrode 111. The sputtered and released gas has positive ionic molecules, so that it is effectively caught and collected by the focusing electrode 116 composed of the getter materials that have substantially the same potential as that of the cathode electrode 112. As a result, the residual gas present in the vacuum chamber can be efficiently captured as not to affect the electron emitting capability of the emitter cones 113.
In the conventional micro vacuum pumps described above, the getters are activated and the activated getters adsorb the gaseous molecules in the vacuum chamber. Hence, active gases including oxygen- and carbon-based gases can be adsorbed, however, inert gases including argon cannot be adsorbed.
Thus, there has been a problem in that the capability of exhausting rare gases, i.e. inert gases, is deteriorated and the quality and performance required of the vacuum pumps cannot be ensured. This means that unstable images, deteriorated luminance, or shorter service life has been observed when driving a CRT, flat panel display, or the like in such a vacuum environment.
Further, in the vicinity of the emitter cones or the protrusions, ionized residual gases such as argon having a high sputtering yield pour down on the negative-electrode protrusions and inevitably damage the protrusions that emit electrons in the getter device. This leads to marked deterioration in the electron emitting property and makes it difficult to retain stable gettering performance with good repeatability over a long period of time.